Traditionally, microphone transmits sound via air; therefore, the transmission is prone to be interfered by external noise which makes the sound quality declined. The traditional electret condenser microphone composes a weightless membrane and a back plate for electret charge. The components of the electret condenser microphone are very sensitive to the external noise; the input sound signal oscillates the metal plate making the distance and the capacitance between the metal plate and the back plate change. Owing to the electret condenser microphone with extremely small capacitance has a high current consumption, the output electric current from the electret condenser microphone should be amplified by dielectrode to an acceptable level to connect with an amplifier. When the microphone receives certain level of sound pressure, it generates voltage at output terminal with which to detect the dB value, and further can be used to measure the sensitivity of a microphone which is proportional to the output voltage value. Another important character of the microphone is its output impedance, which is usually divided to three groups: low impedance (50-1000 ohms), medium impedance (5000-15000 ohms), and high impedance (over 20000 ohms).
Generally speaking, a mobile communication device includes a microphone and powered by a battery. The battery can provide DC power which is more stable and causes less electromagnetic interference than AC power. When the battery of the mobile communication device is exhausted, the battery needs to be charged via an outer charger (for example, travel charger) that transforms the AC power to DC power. When the mobile communication device processes charging and communicating with others simultaneously, the low frequency (50-60 Hz) interference generated by charger would radiate to the surroundings of the mobile communication device by the power cord of the charger; therefore the microphone is easily interfered by electromagnetic wave generated by AC power and makes the user interfered by low frequency (50-60 Hz) noise.
FIG. 1 illustrates a traditional microphone, the negative terminal 12(−) and positive terminal 13(+) are attached to a microphone 6; wherein the negative terminal 12 and positive terminal 13 are made of copper foil and both terminals are for sound output; for example, the Vout shown in the FIG. 1. For the traditional microphone, the copper foil of negative terminal 12 does not shield the positive terminal 13; therefore the positive terminal 13 is easily influenced by an external electromagnetic wave. Further, the area of positive terminal 13 and the one surrounded by the copper foil of negative terminal 12 relate to the level of interference and quality of communication for a microphone.
FIG. 2 shows a traditional microphone, the signals of the negative terminal 12 and the positive terminal 13 are transmitted to an amplifier (not shown) of the mobile communication device by the first conducting wire 26 and the second conducting wire 27 respectively. In virtue of the parallel arrangement of the first conducting wire 26 and the second conducting wire 27, their large exposure area makes the electromagnetic radiating area increase, and makes the microphone more easily interfered by AC electromagnetic wave hence declines the communication quality.
In other word, if a mobile communication device connects to a power supply for transforming AC to DC, the voice output of a microphone is pretty easily interfered by the power noise (50-60 Hz) caused by AC. Therefore, to enhance the communication quality of a mobile communication device, it is necessary to solve the noise interference. Generally, the question is solved by adding an integrated element on the circuit board or improving the design of a power transformer for transforming AC to DC, but these two methods are costly, time consuming and inefficient.
Therefore, the structure of a microphone has become an important factor in affecting the noise interference to the mobile communication device. To solve the shortcoming of the traditional technique, especially utilizing traditional technique for charging a traditional mobile communication device, the present invention provides a new microphone design to effectively reduce the noise interference of a microphone.